Roy E. Hogan

Solar Thermochemical Water-Splitting Ferrite-Cycle Heat Engines

Thermochemical cycles are a type of heat engine that utilize high-temperature heat to produce chemical work. Like their mechanical work producing counterparts, their efficiency depends on the operating temperature and on the irreversibility of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxide (ferrites) working materials. The design concepts utilize two sets of moving beds of ferrite reactant materials in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency-thermal recuperation between solids in efficient countercurrent arrangements. They also provide an inherent separation of the product hydrogen and oxygen and are an excellent match with a high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this paper, the counter-rotating-ring receiver/reactor/ recuperator solar thermochemical heat engine concept is introduced, and its basic operating principles are described. Preliminary thermal efficiency estimates are presented and discussed. Our results and development approach are also outlined.

Carbon dioxide reforming of methane in a solar volumetric receiver/reactor : the CAESAR project

Abstract Solar reforming of methane with CO 2 was successfully demonstrated with a direct absorption receiver/reactor on a parabolic dish capable of 150 kW solar power. The reactor, a volumetric absorber, consisted of a reticulated porous alumina foam disk coated with rhodium catalyst. The system was operated during both steady-state and solar transient (cloud passage) conditions. The total solar power absorbed reached values up to 97 kW and the maximum methane conversion was almost 70%. Receiver efficiencies ranged up to 85% and chemical efficiencies peaked at 54%.

Comparison of a cavity solar receiver numerical model and experimental data

Results from a numerical model of axisymmetric solar cavity receivers are compared with experimental data for tests of a novel test bed receiver in the Saudi National Laboratories solar furnace. The computed energy transfer rates and temperatures are compared with the experimental data for different receiver geometries, aperture sizes, and operating conditions. In general, the agreement between the numerical model and the experimental data is better for the small-to-midsized apertures than for the large apertures. The analysis indicates that for the larger apertures, the convective heat losses are overpredicted. It also suggests that these losses could be better characterized. Sensitivity analyses show that both the total solar energy input rate and the convective heat-loss coefficient significantly affect the receiver thermal performance and that the distribution of the input solar flux significantly affects the temperature distribution in the receiver.

AEETES—A solar reflux receiver thermal performance numerical model

Reflux solar receivers for dish-Stirling electric power generation systems are currently being investigated by several companies and laboratories. In support of these efforts, the AEETES thermal performance numerical model has been developed to predict thermal performance of pool-boiler and heat-pipe reflux receivers. The formulation of the AEETES numerical model, which is applicable to axisymmetric geometries with asymmetric incident fluxes, is presented in detail. Thermal efficiency predictions agree to within 4.1% with test data from on-sun tests of a pool-boiler reflux receiver. Predicted absorber and sidewall temperatures agree with thermocouple data to within 3.3 and 7.3%, respectively. The importance of accounting for the asymmetric incident fluxes is demonstrated in comparisons with predictions using azimuthally averaged variables. The predicted receiver heat losses are characterized in terms of convective, solar radiative, and infrared radiative, and conductive heat transfer mechanisms.

Analysis of Catalytically Enhanced Solar Absorption Chemical Reactors: Part II—Predicted Characteristics of a 100 kWchemical Reactor

The CAtalytically Enhanced Solar Absorption Receiver (CAESAR) is a 100 kWchemecal test reactor currently in operation. This type of high-temperature chemical reactor volumetrically absorbs concentrated solar energy throughout a catalytic porous absorber matrix volume, promoting heterogeneous reactions with fluid-phase reactant species flowing through the absorber. A numerical model of these reactors has been developed to provide guidance in the catalytic matrix design for CAESAR. In the CAESAR reactor, methane is reformed using carbon dioxide and a rhodium catalyst. In addition, the model is being used to evaluate both the reactor performance and test data. This paper presents the thermal and chemical characteristics of the reactor for varying incident solar flux, fluid mass flow, convective heat-transfer coefficient, solar and infrared extinction coefficients, and catalyst loading. Predicted CAESAR performance is based on a prototype absorber and anticipated operating conditions. Model results suggest the mass flux must be proportioned to the incident solar flux radial distribution to prevent unacceptably high local temperatures and to provide a reactor having more uniform exit conditions. Either the catalytic loading or geometric thickness of the absorber should be increased for conversion to approach equilibrium levels. Also, the optical density of the matrix (particularly at the sunlit side of the reactor) should be decreased to distribute solar energy more uniformly in depth and decrease matrix temperatures at the front of the absorber.

Numerical Modeling of Solar Thermo-Chemical Water-Splitting Reactor

Production of hydrogen using solar thermal energy has the potential to be a viable alternative to other hydrogen production methods, typically fossil-fuel driven processes. Thermochemical reactions for splitting water require high temperatures to operate effectively, for which solar is well-suited. Numerical modeling to investigate the concept of a solar-driven reactor for splitting water is presented in detail in this paper for an innovative reactor, known as the “counter-rotating-ring receiver/reactor/recuperator” (CR5) solar thermochemical heat engine that is presently under development. In this paper, details of numerical simulations predicting the thermal/fluid behavior of the innovative solar-driven thermo-chemical reactor are described in detail. These scoping calculations have been used to provide insight into the thermal behavior of the counter-rotating reactor rings and to assess the degree of flow control required for the CR5 concept.Copyright

Photoacoustic sounds from meteors

Concurrent sound associated with very bright meteors manifests as popping, hissing, and faint rustling sounds occurring simultaneously with the arrival of light from meteors. Numerous instances have been documented with −11 to −13 brightness. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which travel time would be several minutes. Concurrent sounds must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer, and transduced into acoustic waves. Previously, energy propagated from meteors was assumed to be RF emissions. This has not been well validated experimentally. Herein we describe experimental results and numerical models in support of photoacoustic coupling as the mechanism. Recent photometric measurements of fireballs reveal strong millisecond flares and significant brightness oscillations at frequencies ≥40 Hz. Strongly modulated light at these frequencies with sufficient intensity can create concurrent sounds through radiative heating of common dielectric materials like hair, clothing, and leaves. This heating produces small pressure oscillations in the air contacting the absorbers. Calculations show that −12 brightness meteors can generate audible sound at ~25 dB SPL. The photoacoustic hypothesis provides an alternative explanation for this longstanding mystery about generation of concurrent sounds by fireballs.

A Two-Phase Model for Solar Thermochemical Water Splitting With FeO/Fe3O4

A two-phase, multi-dimensional, multi-physics computational model based on the finite-element method is presented for simulating the solar thermochemical water splitting process in which hydrogen gas is produced from steam. The model takes into account heat transfer, gas-phase diffusion and advection of neutral species in open channels and through pores of the porous reactant layer, solid-state transport of charged species, and redox chemical reactions. Preliminary results (temperature distribution, velocity field, and species concentration) computed from the gas-phase transport dominating (i.e., resistance to solid-state transport is taken to be negligible) regime are presented to illustrate the utility of the model. Efforts are underway to improve the present first-generation model by incorporating solid-state transport with more realistic kinetic model, internal radiation, reactant-layer support, and temperature-dependent properties.Copyright

MODELING CHEMICAL AND THERMAL STATES OF REACTIVE METAL OXIDES IN A CR5 SOLAR THERMOCHEMICAL HEAT ENGINE.

“Sunshine to Petrol” is a grand-challenge research project at Sandia National Laboratories with the objective of creating a technology for producing feedstocks for making liquid fuels by splitting carbon dioxide (and water) using concentrated solar energy [1]. A reactor-level performance model is described for computing the solar-driven thermochemical splitting of carbon dioxide via a two-step metal-oxide cycle. The model simulates the thermochemical performance of the Counter-Rotating-Ring Receiver/Reactor/Recuperator (CR5). The numerical model for computing the reactor thermochemical performance is formulated as a system of coupled first-order ordinary differential equations describing the energy and mass transfer within each reactive ring and radiative energy transfer between adjacent rings. In this formulation, each of the counter-rotating rings is treated in a one-dimensional sense in the circumferential direction; supporting circumferential temperature and species gradients with assumed negligible gradients in both the radial and axial directions. The model includes radiative heat transfer between adjacent counter-rotating rings, variations in the incident solar flux distribution, heat losses to the reactor housing, and energy of reaction associated with the reduction and oxidation reactions. An overview of the physics included in this first-generation numerical model will be presented. Preliminary results include the circumferential distributions of temperature and species within each of the reactive rings. The computed overall chemical conversion efficiency will be presented for a range of design and operating parameters; including ring speed, carrier ring mass, reactive material loading, radiative emissivity, and differing incident flux distributions.Copyright

Reimagining liquid transportation fuels : sunshine to petrol.

Two of the most daunting problems facing humankind in the twenty-first century are energy security and climate change. This report summarizes work accomplished towards addressing these problems through the execution of a Grand Challenge LDRD project (FY09-11). The vision of Sunshine to Petrol is captured in one deceptively simple chemical equation: Solar Energy + xCO{sub 2} + (x+1)H{sub 2}O {yields} C{sub x}H{sub 2x+2}(liquid fuel) + (1.5x+.5)O{sub 2} Practical implementation of this equation may seem far-fetched, since it effectively describes the use of solar energy to reverse combustion. However, it is also representative of the photosynthetic processes responsible for much of life on earth and, as such, summarizes the biomass approach to fuels production. It is our contention that an alternative approach, one that is not limited by efficiency of photosynthesis and more directly leads to a liquid fuel, is desirable. The development of a process that efficiently, cost effectively, and sustainably reenergizes thermodynamically spent feedstocks to create reactive fuel intermediates would be an unparalleled achievement and is the key challenge that must be surmounted to solve the intertwined problems of accelerating energy demand and climate change. We proposed that the direct thermochemical conversion of CO{sub 2} and H{sub 2}O to CO and H{sub 2}, which are the universal building blocks for synthetic fuels, serve as the basis for this revolutionary process. To realize this concept, we addressed complex chemical, materials science, and engineering problems associated with thermochemical heat engines and the crucial metal-oxide working-materials deployed therein. By projects end, we had demonstrated solar-driven conversion of CO{sub 2} to CO, a key energetic synthetic fuel intermediate, at 1.7% efficiency.